The present invention concerns a piezoelectric motor.
A piezoelectric motor is already known from European Patent Application No. 537 446 in the name of the Applicant. This piezoelectric motor includes a stator mounted, for example by being forcibly driven or bonded onto a stepped arbour which is located on the rotational axis of said motor. This piezoelectric motor further includes a rotor, which is held in axial contact on the stator via support means. A disc-shaped piezoelectric transducer is arranged beneath the stator. This piezoelectric transducer may be formed, for example, by a ceramic element capable of being electrically excited to induce a vibratory movement in the stator. Means for transmitting the vibratory movement to which the stator is subjected are arranged on the rotor, this transmission means being formed by deflectable tongues, which can be deformed elastically.
When subjected to an electric excitation voltage, the piezoelectric element vibrates radially. The vibration of the piezoelectric element induces mechanical stresses in the stator via the effect of which the latter will begin to vibrate vertically, on either side of its rest position. The deformation by deflection of the stator, and thus the essentially linear movement of each point of the stator, are converted into a concomitant rotational movement of the rotor, owing to the elastically deformable deflectable tongues through which the rotor rests, via its external periphery, on the stator. The deflectable tongues, while being stressed by the stator which vibrates axially, bend and induce in the rotor velocity components tangential to the periphery of the rotor.
The piezoelectric motor described hereinbefore has the merit of being able to be miniaturised, of being capable of delivering a significant driving torque, and of having a simple structure able to be series manufactured at low costs. Unfortunately this piezoelectric transducer is only able to rotate in a single direction. This is due to the fact that conversion of the stator""s axial vibratory movement into a rotational movement of the rotor is assured by the aforementioned deflectable tongues, which, with a straight line parallel to the rotational axis of the piezoelectric motor, form an angle which is fixed during the manufacturing operations of said motor and which cannot be modified during operation of the latter. It is thus impossible to have this rotate in the opposite direction. In order to overcome this problem, one could envisage providing the rotor with deflectable tongues alternately having a positive or negative angle of inclination with respect to a straight line parallel to the rotational axis of the motor. However, it is not known how to make the piezoelectric element vibrate to excite only the deflectable tongues oriented in one direction to the exclusion of the tongues oriented in the other direction and vice versa.
There is also known from U.S. Pat. No. 4,918,351 in the name of the Olympus Optical company, a conventional progressive wave piezoelectric motor. This motor includes a plurality of juxtaposed piezoelectric elements bonded onto the lower surface of an element whose role is to amplify the deformation movement of said piezoelectric elements and to avoid any direct contact between these piezoelectric elements and the rotor. The rotor is in contact with the movement amplifier element via a thin friction layer. The orientation of the dipole moments of the electrostrictive elements is reversed from one such element to that arranged just next to it such that, by introducing a phase shift between the excitation voltages of said electrostrictive elements, it is possible to create a progressive deformation wave in the amplifier element. In order to change the rotational direction of the motor, one need only reverse the phase shift between the excitation voltages. The Olympic motor also includes resilient arms which are fixed, at their base, to the stator, and which are in sliding support contact with the electrostrictive disc at their other end. These arms play no role in the rotational movement of the motor and their only purpose, by being resiliently deformed, is to absorb the vibrations generated by the operation of the motor, and thus to prevent these vibrations spreading to the stator.
Although it is able to rotate in both directions, the Olympus progressive wave motor suffers from the fact that it is difficult to guarantee contact with the necessary precision when the motor components are being manufactured. The Olympus progressive wave motor includes, for this reason, a stamped part which compensates for the mechanical tolerances and variations due to wear between the contact elements. The structure of the progressive wave motor is thus rather complicated and is difficult to miniaturise at acceptable costs.
The object of the present invention is to overcome the aforementioned problems of the prior art in addition to others, by proposing a cheap and miniaturisable piezoelectric motor capable of moving in both rotational directions.
The present invention thus concerns a piezoelectric motor of the type including:
a stator;
a rotor capable of moving in rotation in a plan called the mean movement plane perpendicular to a geometrical rotational axis on which the rotor is centred;
coupling means for driving the rotor arranged between the stator and said rotor;
piezoelectric means capable of being electrically excited to impart a vibratory movement onto the coupling means;
transmission means able to transmit the vibratory movement from the coupling means to the rotor in order to drive said rotor in rotation about its axis, and
holding means for applying the rotor onto the stator,
characterised in that the coupling means are arranged freely about the geometrical rotational axis on which they are centred, and in that the coupling means rest on the stator via support means shaped to convert the vibratory movement of the points in the contact region between the coupling means and the rotor into a substantially elliptical forward or backward movement, essentially perpendicular to the plane of the rotor.
Owing to these features, the present invention provides a piezoelectric motor able to rotate forwards or backwards. In order to achieve this object, the piezoelectric motor according to the invention is shaped such that the points of the coupling means in the contact region between said coupling means and the rotor describe a substantially elliptical trajectory, perpendicularly to the plane of the rotor, either in the direct trigonometric direction, or in the opposite direction, such that said rotor can be driven forwards or backwards. The elliptical movement is formed of a substantially circular movement in the plane of the coupling means and an axial vibratory movement of said coupling means. Indeed, assuming that the vibratory movement of the coupling means is in the phase in which the latter are axially deformed downwards at their centre, it is observed that because of the structure of the support means via which the coupling means rest on the stator, the downward axial deformation of said coupling means imparts thereto an angular pivoting movement. Moreover, at the same time that the centre of the coupling means is deformed axially downwards, their external periphery can either be deformed axially upwards or downwards as a function of the frequency of the excitation signal which is applied to the piezoelectric means. Thus, the combination of the angular pivoting movement which arises from the downward axial deformation of the centre of the coupling means and the positive or negative extension movement, i.e. beyond or short of their rest position, of the external periphery of said coupling means, generates an elliptical movement of said external periphery of said coupling means, which are then able to drive the rotor in rotation. Moreover, depending upon whether the centre and the periphery of the coupling means are deformed in phase, i.e. they are both deformed axially downwards during the same half-period of the excitation signal of the piezoelectric means, or in phase opposition, i.e. the centre of the coupling means is deformed by bending downwards while the periphery of said coupling means moves upwards, this generates a reversal of the rotational direction of the rotor.